Environmental control networks or facility management systems are employed in office buildings, manufacturing facilities, and the like, for controlling the internal environment of the facility. The environmental control network may be employed to control temperature, fluid flow, humidity, lighting, boilers, chillers, power, security and similar systems in the internal environment.
For example, in environmental control networks configured to control temperature and air flow, controlled air units such as variable air volume (VAV) boxes and unitary devices (UNT) are located throughout the facility to provide environmentally controlled air to the internal environment. The controlled air is provided at a particular temperature or humidity so that a comfortable internal environment is established and maintained.
The VAV boxes and unitary devices are coupled via duct work to a source of conditioned air, known as an air handling unit (AHU). VAV boxes and unitary devices may include a fan or other device for blowing the controlled air. VAV boxes and unitary devices may also include a damper for regulating the amount of the controlled air provided to the internal environment. The damper is coupled to an actuator, which preferably positions the damper so that appropriate air flow is provided to the internal environment.
In modern systems, a digital controller is typically associated with at least one of the actuator and the damper. The controller receives information related to the air flow and temperature (known as “controlled variables”) in the internal environment and appropriately positions the actuator so that the appropriate air flow is provided to the internal environment.
The AHU also includes a digital controller which may control the supply of cooled air by regulating the flow of chilled water through a cooling coil. The controller regulates the flow of chilled water to the cooling coil by adjusting the position of a valve based on a feedback signal indicative of the temperature of the air discharged from the coil (another “controlled variable”). The feedback signal is generated by a sensor disposed to monitor the controlled variable.
The AHU and VAV controllers use the feedback signals to maintain the controlled variables within certain tolerances of desired levels (known as “setpoints”). For example, the AHU controller attempts to maintain the temperature of the air discharged from the system at a specific level. When the actual temperature of the discharged air deviates from the desired temperature, the controller must appropriately adjust the flow of the chilled water to bring the actual air temperature back in line with the desired air temperature. Thus, if the feedback signal indicates that the actual air temperature is colder than the desired temperature, the controller decreases the flow rate of chilled water to cause the actual temperature of the discharged air to increase. Likewise, if the feedback signal indicates that the actual air temperature is warmer than the desired temperature, the controller increases the flow rate of chilled water to cause the actual temperature of the discharged air to decrease.
An ideal feedback control system would be able to maintain the controlled variable at the setpoint based only on the feedback signal. However, actual feedback control systems require additional inputs known as control parameters that are used by the controller to determine how to control the system based on the feedback signal and the setpoint. Common control algorithms that make use of such control parameters are proportional (P) control, proportional integral (PI) control, and proportional-integral derivative (PID) control. More recently, a pattern recognition adaptive control (PRAC) method has been utilized to automatically determine the values of the control parameters after significant setpoint changes or load disturbances have occurred. One example of an improved PRAC method is disclosed in commonly owned U.S. patent application Ser. No. 10/612,621 (now U.S. Pat. No. 6,937,909).
With any of the forgoing feedback control strategies, however, it can be difficult to maintain the controlled variable precisely at the desired set point for various reasons, including that the appropriate values for the control parameters may change over time as the system is used. For example, the dynamics of a process may be altered by heat exchanger fouling, inherent nonlinear behavior, ambient variations, flow rate changes, large and frequent disturbances, and unusual operations status such as failures, startup and shutdown. The process of adjusting the control parameters of a controller to compensate for such system changes is called retuning. If a controller is not periodically retuned, the control response may become poor. For example, the controlled variable may become unstable or oscillate widely with respect to the setpoint. This can result in inefficient operation as well as increase the maintenance costs due to unnecessary wear of the components.
Significant advances in the art of monitoring the performance of environmental control systems and diagnosing problems therewith have been disclosed in commonly owned U.S. Pat. Nos. 5,555,195 and 5,682,329 (“the '195 and '329 patents”), the entire contents of which are hereby incorporated by reference herein. The '195 and '329 patents disclose diagnostic systems that may be utilized to analyze the performance of devices in an environmental control system such as an HVAC or VAV box. The diagnostic systems disclosed in these two patents advantageously record temperature, air flow, actuator position and other data used in the VAV controllers and generate associated performance indices such as exponentially weighted moving averages (EWMAs). The performance indices may be related to error values, process output values, actuator positions, changes in actuator positions, duty cycles of the actuators, or starts, stops and reversals of the actuators. The calculated and stored performance indices allow building operators to analyze the VAV boxes and controller performance during particular time periods (e.g., commissioning) as well as during the useful lifetimes of the systems.
In addition to monitoring and diagnostic systems such as described above, it is also known to provide alarm/warning systems and data visualization programs to assist building operators with deriving meaningful information from the data that is gathered. However, human operators must typically select the thresholds for alarms and warnings, which can be a daunting task. If the thresholds are too tight, numerous false alarms may be issued. Conversely, if the thresholds are too loose, equipment or system failures can go undetected.
In view of the forgoing, it would be desirable to provide an improved method and apparatus for conveying measured performance indices to building operators. It would be further be desirable to convey performance indicators to building operators for analyzing system performance and diagnosing problems without requiring manual setting of alarm and warning thresholds.